Breast development and tumorigenesis are influenced by hormonal and growth factor signals whose responses are mediated by both intracellular and cell membrane receptors. Estrogen receptor (ER) and progesterone receptor (PR) function in mammary tissue is influenced by the overall receptor composition and by factors that regulate their transactivation potential. For example, some post-transcriptional ER and PR variants, whose expression is increased in certain breast cancer cells, display altered functional activity relative to wild-type receptors and clearly modulate overall receptor function. In addition to steroidal ligands, recent evidence reveals that ERs and PRs can be activated also by certain growth factors that initiate responses at membrane receptors that result in a ligand-independent stimulation of intracellular phosphorylation pathways. We wish to determine whether steroid receptors act as common mediators through which intracellular and membrane signaling pathways converge to regulate target gene expression and thereby influence mammary cellular phenotype, growth pattern, and neoplasia. Our specific objective is to examine the mechanistic contribution of ER and PR composition and receptor activation pathways to the oncogenic refractory state in mammary tissue that is induced by high dose estrogen and progesterone treatment. We propose to: 1) establish the quantitative complement of wild-type and variant ERs and PRs in rat mammary tissue during development, MNU-induced tumorigenesis and after induction of an oncogenic refractory state by early estrogen and progesterone treatment; 2) quantify the functional capacity of ER and PR variants in cells in tissue culture; and 3) determine the ability of growth factor signaling pathways to activate endogenous ER- and PR-dependent gene expression in rat mammary tissue (in vivo) during tumorigenesis and in the refractory phenotype induced by early estrogen plus progesterone treatment.